The present invention is directed to a method and apparatus for achieving a very fine pitch, solderless interconnect between a flexible circuit member and another circuit member, and to an electrical interconnect assembly for forming a solderless interconnection with another circuit member.
It is desirable to probe test each die or device under test (DUT) before the wafer is cut into individual intergrated circuit die or before packaging. Die testing often needs to be performed at high speed or high frequency, for example 100 MHz data rate or higher. The probe cards that support a plurality of probe needles must provide reliable electrical contact with the bonding pads of the DUT. The shank of the probe needle is typically 0.005 inches to 0.010 inches in diameter.
One test probe technique is known as the Cobra system, in which the upper ends of the probe needles are guided through a rigid layer of an insulating material. The upper ends of the individual probe needles are electrically connected to suitable conductors of an interface assembly that is connected to an electrical test system. Each of the needles is curved and the lower ends pass through a corresponding clearance hole in a lower rigid layer or template of insulating material. The bottom ends of the needles contact the bonding pads on the wafer being tested. The length of the probe needles can result undesirable levels of ground noise and power supply noise to the DUT. Additionally, the epoxy or plastic rigid layers have large coefficients of thermal expansion and cause errors in the positioning of the needle probes.
Another draw-back of current test probe technology is that it can often not accommodate fine pitches. For example, wafer probes typically require a target contact area of about 70 micrometers by 70 micrometers. Flip-chip architecture has terminals on the order of 10 micrometers by 10 micrometers, and hence, can not effectively be tested using wafer probe technology. Consequently, integrated circuits in flip-chip architectures can generally be tested only after packaging is completed. The inability to wafer probe integrated circuits used in flip-chip architecture results in production time delays, poor yields and a resultant higher cost.
Many of the problems encountered in testing electrical devices also occur in connecting integrated circuit devices to larger circuit assemblies, such as printed circuit boards or multi-chip modules. The current trend in connector design for those connectors utilized in the computer field is to provide both high density and high reliability connectors between various circuit devices. High reliability for such connections is essential due to potential system failure caused by misconnection of devices. Further, to assure effective repair, upgrade, testing and/or replacement of various components, such as connectors, cards, chips, boards, and modules, it is highly desirable that such connections be separable and reconnectable in the final product.
Pin-type connectors soldered into plated through holes or vias are among the most commonly used in the industry today. Pins on the connector body are inserted through plated holes or vias on a printed circuit board and soldered in place using conventional means. Another connector or a packaged semiconductor device is then inserted and retained by the connector body by mechanical interference or friction. The tin lead alloy solder and associated chemicals used throughout the process of soldering these connectors to the printed circuit board have come under increased scrutiny due to their environmental impact. Additionally, the plastic housings of these connectors undergo a significant amount of thermal activity during the soldering process, which stresses the component and threatens reliability.
The soldered contacts on the connector body are typically the means of supporting the device being interfaced by the connector and are subject to fatigue, stress deformation, solder bridging, and co-planarity errors, potentially causing premature failure or loss of continuity. In particular, as the mating connector or semiconductor device is inserted and removed from the present connector, the elastic limit on the contacts soldered to the circuit board may be exceeded causing a loss of continuity. These connectors are typically not reliable for more than a few insertions and removals of devices. These devices also have a relatively long electrical length that can degrade system performance, especially for high frequency or low power components. The pitch or separation between adjacent device leads that can be produced using these connectors is also limited due to the risk of shorting.
Another electrical interconnection method is known as wire bonding, which involves the mechanical or thermal compression of a soft metal wire, such as gold, from one circuit to another. Such bonding, however, does not lend itself readily to high-density connections because of possible wire breakage and accompanying mechanical difficulties in wire handling.
An alternate electrical interconnection technique involves placement of solder balls or the like between respective circuit elements. The solder is reflown to form the electrical interconnection. While this technique has proven successful in providing high-density interconnections for various structures, this technique does not facilitate separation and subsequent reconnection of the circuit members.
An elastomer having a plurality of conductive paths has also been used as an interconnection device. The conductive elements embedded in the elastomeric sheet provide an electrical connection between two opposing terminals brought into contact with the elastomeric sheet. The elastomeric material must be compressed to achieve and maintain an electrical connection, requiring a relatively high force per contact to achieve adequate electrical connection, exacerbating non-planarity between mating surfaces. Location of the conductive elements is generally not controllable. Elastomeric connectors may also exhibit a relatively high electrical resistance through the interconnection between the associated circuit elements. The interconnection with the circuit elements can be sensitive to dust, debris, oxidation, temperature fluctuations, vibration, and other environmental elements that may adversely affect the connection.
The problems associated with connector design are multiplied when multiple integrated circuit devices are packaged together in functional groups. The traditional way is to solder the components to a printed circuit board, flex circuit, or ceramic substrate in either a bare die silicon integrated circuit form or packaged form. Multi-chip modules, ball grids, array packaging, and chip scale packaging have evolved to allow multiple integrated circuit devices to be interconnected in a group.
One of the major issues regarding these technologies is the difficulty in soldering the components, while ensuring that reject conditions do not exist. Many of these devices rely on balls of solder attached to the underside of the integrated circuit device which is then reflown to connect with surface mount pads of the printed circuit board, flex circuit, or ceramic substrate. In some circumstances, these joints are generally not very reliable or easy to inspect for defects. The process to remove and repair a damaged or defective device is costly and many times results in unusable electronic components and damage to other components in the functional group.
Multi-chip modules have had slow acceptance in the industry due to the lack of large scale known good die for integrated circuits that have been tested and burned-in at the silicon level. These dies are then mounted to a substrate which interconnect several components. As the number of devices increases, the probability of failure increases dramatically. With the chance of one device failing in some way and effective means of repairing or replacing currently unavailable, yield rates have been low and the manufacturing costs high.
U.S. Pat. No. 5,252,916 (Swart) discloses a fluid-activated fixture for printed circuit boards. An electrically conductive barrel 22 (also referred to as an eyelet) is press fitted into each bore 20. Separate test probes 24 are movably mounted in each of the barrels 22. A flex circuit 30 is laminated to a bottom surface of the upper probe plate 16. Each barrel 22 has an outer flange that pierces a circuit trace and seats the flange to the circuit trace to form an electrical connection. The test probes 24 are movable axially in their respective barrels freely and under gravity. The test probes slide on the inside of the barrels 22 for making sliding electrical contact. The barrels 22 press fit into the support plate 16 make electrical contact with the flexible circuit 30. The first ends of the test probes 24 are supported by a flexible elastomeric diaphragm 42.
U.S. Pat. No. 4,118,090 (Del Mei) discloses a plurality of movable electrically conductive contacts 10 slidably located in cylindrical apertures 14 in a locating element 16. The contacts 10 are each attached to an electrically insulating elastomeric element 18 which has been bonded to the contacts by molding the element 18 about the contacts 10. A retaining member 20 is provided on the opposite side of the elastomeric element 18 to the locating element and traps the elastomeric element between itself and the locating element.
WO 98/13695 discloses a connector apparatus in which an anisotropic compliant conductive interposer 214 electrically couples contact elements 208 to contact pads 322 on an interface board 320. Stop rings 312 retain the contact elements 208 in the guide plate 108. The anisotropic interposer 214 is comprised of an elastomeric sheet 350 with a plurality of conductors 352. The interposer 214 serves as a pass-through for electrical signals between the contact elements 208 and the interface board 320.
U.S. Pat. No. 5,723,347 (Hirano et al.) discloses a probe structure with a plurality of conductive contacts formed on a film stretched across a plurality of cavities in a substrate. The cavities and the conductive contacts are aligned to one another and both match the positions of selected I/O pads on the device to be probed.
U.S. Pat. No. 5,412,329 (Jino et al.) discloses a probe card having a supporting plate, a flexible printed circuit base including a flexible film base material supported by the supporting plate, circuits printed on the film base material being connected electrically to a tester, contracters connected electrically to the printed circuits and adapted to be brought into contact with the pads in equally corresponding relation, and a cushioning medium designed so as to back up a section in which the contactors are mounted. When the contactors are brought into contact with the pads, individually, the cushioning medium undergoes an elastic deformation, so that the contact between the contactors and the pads is improved.
EP 0 310 302 discloses a test socket for testing chips and chips on tape wherein the test socket is formed on a heat resistant dielectric film having contact pads and connector pads joined by metallic circuit traces and which film is wrapped on a compliant pad. The connector end of the tape is joined to a circuit board by a conductive tape and maintained in contact by the compliant pad. A frame registers the chip with the contact area of the tape.
The present invention is directed to a method and apparatus for achieving a very fine pitch interconnect between a flexible circuit member and another circuit member with extremely co-planar electrical contacts that have a large range of compliance. The second circuit member can be a printed circuit board, another flexible circuit, a bare-die device, an integrated circuit device, an organic or inorganic substrate, a rigid circuit and virtually any other type of electrical component.
The present invention is also directed to an electrical interconnect assembly comprising a flexible circuit member electrically coupled to an electrical connector in accordance with the present invention. The present electrical interconnect assembly can be used as a die-level test probe, a wafer probe, a printed circuit probe, a connector for a packaged or unpackaged circuit device, a conventional connector, a semiconductor socket, and the like.
The present method includes preparing a plurality of through holes extending between a first surface and a second surface of a housing. Each of the through holes defines a central axis. A plurality of elongated electrical contacts are positioned in at least some of the through holes and oriented along the central axis. The electrical contacts have first ends that extend beyond the first surface. The electrical contacts are retained in the through holes by a variety of techniques. The first ends of the electrical contacts are electrically coupled to contact pads or terminals on a flexible circuit so that the second ends of the electrical contacts extend beyond the second surface. The second ends of the electrical contacts are then free to electrically couple with a second circuit member. A resilient member controls movement of the electrical contacts along their respective central axes within the housing.
The step of retaining the electrical contacts in the through holes can be achieved by interposing a compliant encapsulating material between a portion of the through holes and a portion of the electrical contacts, surrounding a portion of the electrical contacts with an encapsulating material along one of the surfaces of the housing, bonding the first end of the electrical contacts to the terminals on the flexible circuit, and/or positioning a compliant material along a surface of the flexible circuit opposite the terminals. A back-up member may optionally be positioned behind the compliant material. In one embodiment, the compliant encapsulant elastically bonds the electrical contacts to the housing.
In one embodiment, the step of positioning the plurality of electrical contacts includes applying a solder mask material or comparable dissolvable/removable material along the first surface. The solder mask material and a portion of the electrical contacts extending above the first surface are planarized. When the solder mask is removed, the electrical contacts have precisely formed end surfaces that extends above the first surface of the housing. The resilient member can optionally be applied to the electrical contacts either before application of the solder mask or after removal of the solder mask.
The ends of the electrical contacts can be modified by a variety of techniques, such as etching, grinding, abrasion, ablation or the like. The ends of the electrical contacts can also be modified to have a shape that facilitates engagement with various structures on the flexible circuit member or the second circuit member. The second ends of the electrical contacts can be configured to engage with another flexible circuit, a ribbon connector, a cable, a printed circuit board, a bare die device, a ball grid array, a land grid array, a plastic leaded chip carrier, a pin grid array, a small outline integrated circuit, a dual in-line package, a quad flat package, a flip chip, a leadless chip carrier, and a chip scale package.
The first ends of the electrical contacts are electrically coupled to the flexible circuit bonding pads using a variety of techniques, such as a compressive force, solder, wedge bonding, conductive adhesives, solder paste, ultrasonic bonding, wire bonding, or a combination thereof. In one embodiment, the flexible circuit is bonded to the first surface of the housing with an adhesive.
The electrical connector in accordance with the present invention includes a housing with a plurality of through holes extending between a first surface and a second surface. A plurality of elongated electrical contacts are positioned in the through holes and oriented along the central axis. The first ends of the electrical contacts are electrically coupled to the terminals on the flexible circuit. The second ends extend beyond the second surface of the housing to couple electrically with the second circuit member. A resilient member controls movement of the electrical contacts along their respective central axes. The resilient member can be an encapsulating material interposed between a portion of the through hole and a portion of the electrical contacts, an encapsulating material surrounding a portion of the electrical contacts along one of the surfaces of the housing, the flexible circuit bonded to the contacts, a singulated terminal on the flexible circuit, and/or a compliant material positioned along a surface of the flexible circuit opposite the terminals.
The electrical contacts can be a multi-layered construction or a homogenous material. The electrical contacts may have a cross-sectional shape of circular, oval, polygonal, or rectangular. The electrical contacts can have a pitch of less than about 0.4 millimeters and preferably a pitch of less than about 0.2 millimeters.
The present invention is also directed to an electrical interconnect assembly comprising a flexible circuit bonded to the first ends of the electrical contacts in the housing. A resilient member controls movement of the electrical contacts along their respective axes. The second ends of the electrical contacts are free to engage with a variety of second circuit members, or to operate as test probes for testing various electrical components.